Energy storage devices are traditionally electrochemical accumulators operating on the principle of electrochemical cells able to deliver an electric current owing to the presence, in each of them, of a pair of electrodes (a positive electrode and a negative electrode, respectively) separated by an electrolyte, the electrodes comprising specific materials able to react according to an oxydoreduction reaction, in return for which electrons are produced at the source of the electric current and ions are produced that will circulate from one electrode to another through an electrolyte.
The accumulators adhering to this principle that are currently the most used are:                Ni-MH accumulators using metal hydride and nickel oxyhydroxide as electrode materials;        Ni—Cd accumulators using cadmium and nickel oxyhydroxide as electrode materials;        Acid-Lead accumulators using lead and lead oxide PbO2 as electrode materials; and        lithium accumulators, such as lithium-ion accumulators, traditionally using, in whole or in part, lithium-bearing materials as electrode materials.        
Because lithium is a particularly light solid element having the lowest electrochemical potential, thereby allowing access to an interesting specific energy density, lithium accumulators have greatly supplanted the other aforementioned accumulators due to the continuous improvement in the performance of Li-ion accumulators in terms of energy density. Indeed, lithium-ion accumulators make it possible to obtain specific and volume energy densities (which may be greater than 180 Wh·kg−1) significantly greater than those of Ni-MH and Ni—Cd accumulators (which may go from 50 and 100 Wh·kg−1) and Acid-lead (which may go from 30 to 35 Wh·kg1). What is more, Li-ion accumulators may have a nominal cell voltage greater than that of other accumulators (for example, a nominal voltage of approximately 3.6 V for a cell implementing the LiCoO2/graphite pair as electrode material versus a nominal voltage of approximately 1.5 V for the other aforementioned accumulators).
Due to their intrinsic properties, Li-ion accumulators have therefore proven particularly interesting for fields where autonomy is a crucial criterion, as is the case in the fields of computers, video, telephones, transportation, such as electric and hybrid vehicles, or the medical, spatial, microelectronics fields. However, the performance of lithium-ion accumulators technology is reaching its limits today.
Currently, a new lithium-based accumulator technology is being presented as a promising alternative, this technology being the lithium/sulfur technology, in which the positive electrode comprises, as active material, elementary sulfur or a derivative of sulfur, such as lithium sulfide or lithium polysulfide.
The use of sulfur as active material for a positive electrode is particularly attractive, since sulfur has a very high theoretical specific capacity that may be up to 10 times greater than that obtained for conventional positive electrode materials (approximately 1675 mAh/g instead of 140 mAh/g for LiCoO2). What is more, sulfur is abundantly present on the planet and is therefore characterized by low costs. Lastly, it has a low toxicity. All of these qualities contribute to making it particularly attractive for large-scale use, in particular for electric vehicles, especially given that lithium/sulfur accumulators may make it possible to reach specific energy densities from 300 to 600 Wh·g−1.
From a structural perspective, a lithium/sulfur accumulator battery comprises at least one electrochemical cell including two electrodes based on different materials (a positive electrode comprising elementary sulfur as active material and a negative electrode comprising metal lithium as active material), between which an electrolyte is arranged.
More specifically, the positive electrode is traditionally made from a composite material comprising elementary sulfur and non-electroactive additives, such as an electronic conductive additive used to improve the electronic conductivity of the electrode and a binder, for example, a polymer binder to provide the cohesion between the different components of the composite material.
From a functional perspective, the reaction at the origin of the production of current (i.e., when the accumulator is in discharge mode) involves an oxidation reaction of the lithium at the negative electrode that produces electrons, which will power the outside circuit to which the positive and negative electrodes are connected, and a reduction reaction of the sulfur at the positive electrode.
Thus, explicitly, in the discharge process, the overall reaction is as follows:S8+16Li→8Li2Swhich is the sum of the reduction reaction of the sulfur at the positive electrode (S8+16e−→8S2−) and the oxidation reaction of the lithium at the negative electrode (Li→Li++e−).
It is understood that the reverse electrochemical reactions occur during the charging process.
As emerges from the equation above, the reaction involves the exchange of 16 electrons, which justifies the high specific capacity of the sulfur (1675 mAh·g−1).
From a mechanistic perspective, and without being bound by the theory, in the initial state (i.e., when the battery is in the fully charged state), the active material, which is elementary sulfur, is present in solid state in the positive electrode. During the reduction of sulfur, i.e., during the discharge, the cyclical molecules of sulfur are reduced and form linear chains of lithium polysulfide, with general formula Li2Sn, with n being able to go from 2 to 8. Since the starting molecule is S8, the first compounds formed are the long-chain lithium polysulfides, such as Li2S8 or Li2S6. These lithium polysulfides being soluble in the organic electrolytes, the first discharge step therefore consists of solubilizing the active material in the electrolyte, and producing long-chain lithium polysulfides in solution. As the sulfur reduction continues, the chain length of the polysulfides is gradually reduced, and compounds such as Li2S5, Li2S4 or Li2S2 are formed in solution. Lastly, the final reduction product is lithium sulfide (Li2S), which is insoluble in the organic electrolytes. Thus, the final step of the sulfur reduction mechanism consists of the precipitation of the sulfurated active material.
This mechanism may be correlated to the discharge profile illustrated in FIG. 1, which shows a graph illustrating the evolution of the potential E (in V) as a function of the capacity C (in u.a.).
Indeed, in this profile, the first plateau may be attributed to the formation of long lithium polysulfide chains, while the second plateau corresponds to the reduction of the size of the sulfurated chains, until passivation of the positive electrode. Indeed, the compound at the end of discharging Li2S, like the elementary sulfur, are insulating materials which, when they precipitated at the end of charging or discharging, drastically increase the resistance of the accumulator, and therefore its polarization.
This atypical operation causes many difficulties, which may hinder the large-scale marketing of lithium/sulfur accumulators. In particular, the insulating nature of the active material requires that it be associated with an electronic conductor having a sufficient developed surface to accommodate all of the active material and delay passivation of the electrode. It has in fact been possible to demonstrate that the discharge capacity is greatly related to the specific positive electrode surface accessible to the soluble and insoluble sulfurated species. To that end, positive electrodes have been proposed having a large specific surface, as described in FR 2,979,755, which in particular proposes positive electrodes including a porous electronic conductive substrate, in particular, this substrate being able to assume the form of a metal or carbonaceous foam, the porous structure making it possible to receive a large quantity of sulfurated species at the end of charging and discharging, which makes it possible to improve the discharge capacity. Furthermore, in FR 2,979,755, the active material of the positive electrode is introduced into the electrolyte in the form of lithium polysulfide (which thus constitutes a catholyte), which does not require the preparation of a composite positive electrode, as traditionally used in the lithium-based accumulators, i.e., with a current collecting substrate on which an ink is generally deposited comprising the active material, a binder, for example, polymeric, and an electronic conductive additive. Not using a composite positive electrode makes it possible to avoid a decrease in the capacity during cycles. Indeed, in the context of lithium-sulfur batteries, the successive dissolution and precipitation cycles of the active material at the positive electrode impose significant mechanical stresses thereon. As a result, the morphology of composite electrodes may therefore change substantially upon each cycle, which creates a loss of specific surface, and a concomitant loss of practical discharge capacity.
In light of what exists, the authors of the present invention have proposed to develop a new type of lithium-sulfur positive electrode that has a large specific surface, and furthermore offers good accessibility of its surface to the active material and the electrolyte.